Alfalfa weevil is starting to show up as the weather warms across the West. Larval feeding on folded leaves can heavily damage stem terminals, according to Assefa Gebre-Amlak, Colorado State University Extension specialist. In a recent report, Gebre-Amlak noted that initial damage from larval feeding is not always clearly visible.

For scouts, that means the overlapping foliage of the stem terminals should be unfolded to detect feeding damage. Third and fourth larval instars can cause the most economic damage this time of year. Gebre-Amlak notes initiating sampling at the peak occurrence of second instars can provide adequate sampling prior to economic weevil populations appearing.

In his report, Gebre-Amlak notes: "Field damage can be recognized on heavily infested stands as a grayish or frostlike appearance due to the dried, defoliated leaves. At high weevil densities, foliage can be stripped, leaving only skeletonized and ragged leaf fragments and stems."

With extreme weevil population levels, Gebre-Amlak says yield losses can reach as high as 40% in a standing hay crop. And damage may also reduce hay quality with leaf loss, leaving only lower-quality stems.

Damage to regrowth buds can also occur when plants break dormancy and after first cutting. Larval feeding on post-first-cut regrowth after first cutting may be concentrated in strips coinciding with windrow locations, especially if the first cutting was taken early due to heavy weevil infestation. If larvae survive in the windrows they can return to feed on regrowth, which can also cut yields.

Monitoring and management
Gebre-Amlak says sweeping sampling using a standard 38 cm-diameter net is most efficient for estimating larvae populations. Taking 10, 180-degree sweeps while the sampler is walking the field, followed by counting the number of larvae per sweep, is a start. Then repeat the procedure, taking a minimum of three samples for fields up to 20 acres, four samples for fields up to 30 acres and five samples for larger fields. "The simple economic threshold for a sweep sample is 20 larvae per sweep," Gebre-Amlak says.

A second step is to take three six-stem samples in fields from 1 to 19 acres, four samples in fields 20 to 29 acres, and five samples in fields 30 acres and larger. The economic threshold here is 1.5 to 2 larvae per stem. For calculating detailed economic threshold, check the High Plains IPM guide at highplainsipm.org.

There are two management choices — cultural control and chemical control.

For cultural control, Gebre-Amlak recommends an early first harvest if an economic infestation is not detected until late in the growth of the first cutting. "Harvesting alfalfa in an immature stage provides good control of larvae for the first crop," he says.

Rapid removal of hay accelerates larval mortality due to desiccation by direct sunlight. An early first cutting tends to cure more rapidly because lighter windrows dry faster, and forage quality is enhanced by higher crude protein and lower fiber content.

Added steps should be taken to ensure surviving larvae do not cause economic damage to regrowth. If larval survival under windrows is high and baling is delayed — due to rainfall — damage to regrowth could be enhanced. "Regrowth should be inspected at a height of 1 to 2 inches to determine larval density," he says.

For chemical control, high damage as harvest approaches may require an early harvest or a rescue insecticide treatment. "Use care in apply insecticide when alfalfa is approaching bloom," he advises, noting that when the plant is in bloom, there's a risk to beneficial pollinators.

He also recommends checking the waiting period before harvest for different insecticide treatments. "Generally, harvest or insecticide applications should be taken before bloom if weevils are a problem," Gebre-Amlak says.

Source: Colorado State University